1. Field of the Invention
The present invention relates to a polarization mode dispersion compensator for analyzing signal distortion due to polarization mode dispersion in optical transmission systems and therefore providing a feedback signal to adjust adaptive optics used for mitigating polarization mode dispersion.
2. Description of the Related Art
The higher the bit rate of an optical transmission system, the more a specific amount of polarization mode dispersion of an optical fiber distorts the transmitted signal.
Due to polarization mode dispersion, the two modes in a so called single-mode fiber propagate with different velocities. An initial pulse splits its energy into the two modes. The two modes experience a differential delay during propagation. This leads to pulse spreading at the end of the fiber. The more the differential delay between the two modes is in the order of the bit duration, the more neighboring pulses will overlap, which leads at least to an increasing bit-error rate or makes it even impossible to differentiate the pulses. Polarization mode dispersion is due to internal birefringence (e.g. fiber core geometry irregularities) or externally induced birefringence (e.g. bending, squeezing, etc.). Because in a long single-mode fiber, polarization mode coupling occurs at randomly varying locations with randomly fluctuating strength due to e.g. environmental changes like temperature, polarization mode dispersion itself varies over time. It is well known, that the instantaneous differential group delay between the principal states of polarization follows a Maxwellian probability density function. The mean of the Maxwellian distributed instantaneous differential group delay is known as the average differential group delay, or the polarization mode dispersion value (PMD) of the fiber. The polarization mode dispersion value is, for long single-mode fibers with high polarization mode coupling, proportional to the square root of the fiber length.
To mitigate signal distortion due to polarization mode dispersion, optical elements introducing a similar amount of differential group delay as in the fiber but with an opposite sign, can be placed at the end of the fiber. Due to the random nature of the instantaneous differential group delay and the principal states of polarization in a long optical fiber, the optical elements used for compensating polarization mode dispersion must be adaptively adjusted to the momentary fiber conditions. A closed loop design, polarization mode dispersion compensator consequently consists of:    1. adaptively adjustable optical elements (adaptive optics)    2. distortion analyzer    3. control logic as depicted in FIG. 1.
In FIG. 1, the distortion analyzer 12 provides a measure of signal distortion for the control logic 13 to adaptively adjust the adaptive optics 11, such that they best match the momentary polarization mode dispersion conditions of the optical fiber.
Beside methods like for example spectral hole burning (SHB), direct eye-opening analyzing, etc., the degree of polarization (DOP) can be used for analyzing signal distortion due to polarization mode dispersion. For those who are skilled in the art, it is well known that a light beam experiences depolarization if the coherence length, which is inversely proportional to the spectral width, is in the order of the differential group delay. The higher the differential group delay becomes compared to the coherence length, the more the beam gets depolarized and its degree of polarization decreases. This well known physical effect is straightforward to be used as a feedback signal to adaptively control optical elements of a polarization mode dispersion compensator. Derivation of the depolarization of an optical signal due to fiber anisotropies as a function of signal spectrum (bandwidth, form), differential group delay and state of input polarization is shown in the following reference.
“Degree of Polarization in Anisotropic Single-Mode Optical Fibers: Theory”, Jun-ichi Sakai, SusumuMachida, Tatsuya Kimura, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, pp. 488–495, 1982
Compared to spectral hole burning, measuring directly the eye-opening or bit-error rate detection, the advantages of using degree of polarization as a feedback signal for adaptive polarization mode dispersion compensation are:    1. independent of bit rate    2. applicable to any modulation format without requiring modifications    3. insensitive to chromatic dispersion, such that degree of polarization provides a good measure of signal distortion due to only polarization mode dispersion
Depicted in FIG. 2 are as a function of instantaneous differential group delay the degree of polarization and Q-penalty of a transmitted signal, non-return to zero (NRZ) format modulated with a bit rate of 48 Gbit/s. The Q-penalty is defined here as:
                              Q          ⁢                      -                    ⁢          penalty                =                              20            ·            log                    ⁢                                                    Eye                ⁢                                  -                                ⁢                opening                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                received                ⁢                                                                  ⁢                signal                                            Back                ⁢                                  -                                ⁢                to                ⁢                                  -                                ⁢                back                ⁢                                                                  ⁢                eye                ⁢                                  -                                ⁢                opening                                      .                                              (        1        )            
For reference, also shown in FIG. 2 is the power of the 24 GHz (half the bit rate) spectral component as a function of instantaneous differential group delay. The spectral component at half the bit rate has been proved to show the strongest dependence on instantaneous differential group.
Contrary to the degree of polarization which shows only one maximum if the instantaneous differential group delay vanishes, the 24 GHz spectral component shows a periodic behaviour. Therefore, in cases where the instantaneous differential group delay is expected to exceed on bit duration, at least one more spectral component, namely the 12 GHz (quarter of the bit rate) must be additionally tested to avoid an ambiguity.
The details of degree of polarization and the power of spectral components at 24 GHz (half the bit rate), 12 GHz (quarter of the bit rate) and 6 GHz (eighth the bit rate) are depicted in FIG. 3 for small values of the instantaneous differential group delay.
Of critical importance in the application of degree of polarization as a feedback signal in an adaptive polarization mode dispersion compensator, is the accuracy (particularly the variance of the measured degree of polarization with the input state of polarization) with which the degree of polarization can be measured.
As can be seen from FIG. 2, if for example a Q-penalty of 0.5 dB must not be exceeded, the dynamic range of the degree of polarization is ±5%. The uncertainty, with which the degree of polarization is measured, must therefore not exceed 5%. An adaptive control algorithm of a polarization mode dispersion compensator needs to sample the degree of polarization in the environment of the optimum point in order to track the randomly varying differential group delay and principal states of polarization. Because of the required sampling by slightly mismatching the optical elements with respect to the polarization mode dispersion conditions in the optical fiber, the required accuracy with which the degree of polarization can be measured is much more stringent.
A common approach for realizing the required accuracy is to mechanically align the optical components of a polarimeter in a distortion analyzer with high accuracy to predefined angles. However, this is a very tedious and therefore cost intensive way of construction.